1. Field of the Invention
The present invention generally relates to an imaging device, and more particularly to an imaging device which is applicable to reading optical systems of copy machines, facsimile machines and the like, an optical system of a reading scanner having a CCD sensor and a equimagnification sensor and optical systems of an optical printing head and a self-scanning type optical printing head.
2. Description of the related Art
In the recent years, it is required to miniaturize optical equipment, such as a copy machine and an optical printer head. To satisfy this requirement, a reading optical system and/or a writing optical system of the optical equipment have to be miniaturized. Thus, an equimagnification imaging optical system in which a distance between an object and an image can be strongly reduced is under investigation. The equimagnification imaging optical system is defined as an optical system which forms an image having the same size as an object.
A description will now be given of an example of the equimagnification imaging optical system having a conventional configuration. FIG. 1 illustrates the equimagnification imaging optical system having a conventional configuration. Referring to FIG. 1, a roof mirror lens array 103 is formed as the equimagnification imaging optical system. The roof mirror lens array 103 has a lens array 101 and a roof mirror array 102. The lens array 101 is formed of a plurality of lenses 104 which are arranged in line perpendicular to a a drawing plane of FIG. 1. the lenses 104 are optically equivalent to each other. The roof mirror array 102 is formed of a plurality of roof mirrors 106. The roof mirrors 106 are arranged in line so that each of the roof mirrors 106 faces one of the lenses 104. Each of the roof mirrors 106 has a ridge line 105. The ridge line 105 is perpendicular to a direction in which the roof mirrors 106 are arranged and an optical axis of each of the lenses 104. A stop member (not shown) is provided between the lens array 101 and the roof mirror array 102 so that imaging systems, each of which is formed of one of the lenses 104 and a corresponding one of the roof mirrors 106, are separated from each other.
A reading position P1 of an original 107 is set at a position which is not on the optical axis of each of the lenses 104 and corresponds to a finite slit height position. Light reflected from the reading position P1 of the original 107 passes through the each of the lenses 104 so that the light formed of parallel rays. The parallel rays travels to a corresponding one of the roof mirrors 106 and are reflected by the corresponding one of the roof mirrors 106 in the same direction. The light reflected by each of the roof mirrors 106 travels through a corresponding one of the lenses 104 again and is then focused on an imaging position P2 which is optically conjugate to the reading position P1. The position P2 is, for example, on a surface of a CCD sensor 108.
An prism lens array is disclosed in Japanese Patent Publication No.61-2929. Into this inprism lens array, a lens array and a roof mirror lens array are integrated. In the same manner as the roof mirror lens described above, a reading position is set at a position corresponding to a finite slit height position. The light reflected at the reading position travels through each of lenses and is then reflected by each of roof prisms twice. The light reflected by the each of the roof prisms travels through a corresponding one of the lenses again and is focused on an imaging position which is optically conjugate to the reading position.
A roof mirror lens array which is the equimagnification imaging optical system is disclosed in Japanese Laid-Open Patent Application No.57-37326. Into this roof mirror lens array, a lens array, a roof mirror array and a stop member are integrated the lens array has lenses which are optically equivalent to each other. The lenses are arranged in line. The roof mirror array has roof mirrors. Each of the roof mirrors faces one of the lenses and has a ridge line. The ridge line is perpendicular to a direction in which the lenses are arranged and to an optical axis of each of the lenses. The stop member is provided between the lens array and the roof mirror array to separate imaging optical systems each of which is formed of a corresponding one of the lenses and a corresponding one of the roof mirrors. The roof mirror lens array may be used to read images and for exposure of a photosensitive member.
In each of the imaging devices as described above, a single imaging system is formed of a lens of the lens array and a roof mirror of the roof mirror array. An aperture of the stop member is provided between corresponding lens and roof mirror to optically separate the imaging system from adjacent imaging systems. In this type of the imaging device, the light travels and returns through the lens. Thus, it is not possible to locate the reading position and the imaging position at the same position. The light rays travels along the optical axis are separated to an object (the original) side rays and imaging point side rays. Thus, the reading position and the imaging position have to be set based on a finite slit height position. That is, the reading position P1 is set at a finite height position in a direction parallel to the ridge line 105 of each of the roof mirrors 106. The imaging position P2 is set at the finite height position in the reverse direction.
Since the amount of separation of the light rays is limited, separation mirrors 109(1) and 109(2) are used to set the reading position P1 and the imaging position P2 as shown in FIG. 1. The light traveling from the reading position P1 is reflected by the separation mirror 109(1) and travels to a corresponding one of the lenses 104. The light passing through each of the lenses 104 is reflected by the separation mirror 109(2) and focused on the imaging position. Each of the separation mirrors 109(1) and 109(2) is a rectangular plane mirror which expands in a direction perpendicular to the drawing plane of FIG. 1. Each of the separation mirrors 109(1) and 109(2) are arranged so as to be inclined by 45.degree. with respect to a plane including optical axes .phi. of the lenses 104 of the lens array 101.
In the conventional imaging device having a roof mirror lens or a roof mirror lens array, the light passes through the same lens 104 twice, and the reading position P1 (a reading plane) and the imaging position P2 (an imaging plane) are located in the opposite sides with respect to the optical axis .phi. of the lens 104. The separation mirrors 109(1) and 109(2) are provided in optical paths between the reading position P1 and the lens 104 and between the lens 104 and the imaging position P2.
The surfaces of each roof mirror and the separation mirrors 109(1) and 109(2) are provided with reflecting films which are formed of high reflecting material, such as aluminum (Al), by a vacuum evaporation process. The reflectivity of each of the reflecting films is about 90%. In the imaging device having the above structure as shown in FIG. 1, there are two reflecting surfaces of each of the roof mirrors 106 and two reflecting surfaces of the respective separation mirrors 109(1) and 109(2). Thus, the total reflectivity of is about 66%. The loss of the amount of light in the imaging device is large. In addition, in the conventional case, the light pass through the same lens 104 twice, so that the reading position P1 and the imaging position P2 have to be adjacent and to be symmetrical to each other with respect to the optical axis .phi.. Thus, stray light, such as reflected light from the surface of the lens 104 and from surfaces other than the reflecting surface of the roof mirror 106, may be incident on the imaging position P2 at a high possibility. Such stray light affects characteristics of optical images. In general, the contrast and the resolution of the optical images deteriorate.
Further, FIG. 2 illustrates an essential part of another example of the conventional imaging device. Referring to FIG. 2, the imaging device has a lens array 121 and a roof mirror array 122. Each of roof mirrors of the roof mirror array 122 has a ridge line portion 122a between arranged optical axes. A roof mirror lens array is formed of the lens array 121 and the roof mirror array 122.
Each of the roof mirrors of the roof mirror array 122 has two reflecting surfaces which are connected to each other at an angle of 90.degree. so that the ride line portion 122a is formed. However, light L' which is obliquely incident on each lens of the lens array 121 is reflected by a corresponding one of the roof mirrors twice and then ejected from an adjacent lens. That is, the light L' obliquely incident on an optical system is ejected from an adjacent optical system in the imaging device.
In addition, an example of a conventional imaging device using a roof mirror lens array is shown in FIG. 30. In the imaging device, a lens array and a roof mirror array are integrated. Referring to FIG. 30, the imaging device has a lens array 221 and a roof mirror array 222. In the roof mirror array 222, a ridge line portion 223 is in a boundary between reflection surfaces 224. The lens array 221 has lenses R1 (R2) each of which has an optical axis .phi.1.
In the imaging device having the above structure, for example, light from an original passes through the lens RI and is reflected twice by the reflection surfaces 224 of the roof mirror array 222. The reflected light then passes through the lens R1 (R2) of the lens array 221 and is projected onto an imaging surface so that an equimagnification erect image of the original is formed.
Further, another conventional imaging device has been proposed in Japanese Patent Publication No.5-35245. The proposed imaging device is shown in FIGS. 31A, 31B and 31C. The imaging device is referred to as a roof prism lens array imaging device. FIG. 31A is a side view of the roof prism lens array imaging device, FIG. 31B is a view thereof from a direction B shown in FIG. 31A and FIG. 31C is a view thereof from a direction A shown in FIG. 31A. Referring to FIGS. 31A, 31B and 31C, the imaging device has a roof prism lens array 226. In the roof prism lens array 226, a ridge line portion 228 is in a boundary between reflection surfaces 227.
In the imaging device having such a structure, the light from an original (P1) passes through the lens RI and is reflected twice by the reflection surfaces 227. The reflected light is then emitted from the lens R2 and projected onto an imaging surface P2 so that an equimagnification erect image of the original is formed.
In addition, Japanese Laid-Open Patent Application No.7-35998 discloses an imaging device having a roof mirror lens array which is an equimagnification imaging system. In the imaging device, two or more roof mirrors are provided with respect to each of lenses of the lens array. FIG. 32 shows such an imaging device. The roof mirror array is arranged so that a valley of the roof mirror array is located at a center between optical axes of lenses of the lens array. In the imaging device shown in FIG. 32, when light L1 is incident on a reflection surface of the roof mirror array close to an adjacent part, the reflected light of the incident light L1 is emitted as ghost light from the lens array.
FIGS. 33A and 33B illustrates a state in which cross-talk light is generated in a conventional equimagnification imaging system. FIG. 33A shows an example in which light L1 is incident on the device obliquely and is reflected once by a prism or a roof mirror, so that cross-talk light is generated. FIG. 33B shows an example in which light L1 is reflected twice by reflection surfaces and emitted from a lens other than a Elens from which the light should be emitted, so that an image is formed by cross-talk light at a position other than a position at which an image should be formed.
In such imaging devices, if light from an original is incident on an objective lens at the front thereof, an image is correctly formed on an imaging surface. However, as shown in FIG. 33A, in a case where the light L1 is incident on the device obliquely, the light L1 passes through the lens R and is reflected once by a reflection surface of the roof prism or the roof mirror and emitted from the device. As a result, ghost light is generated. In addition, as shown in FIG. 33B, the light L2 is reflected twice by the reflection surfaces and emitted from a lens adjacent to a lens from which the light L2 should be emitted. As a result, the cross-talk light is generated.
Further, FIG. 34 shows an example of a roof prism lens array imaging device. The roof prism lens array imaging device has a roof prism lens. A ridge line portion 111 is in a boundary between reflection surfaces 112. A groove 113 is formed between reflection surfaces (Dach surfaces). Due to the groove 113, stray light can be prevented from generating in the device.
However, if the light is incident on the lens surface obliquely, the light is reflected by a boundary between the groove 113 and the surface of the prism. The light is further reflected twice by the reflection surfaces 112 and is emitted from the lens R, so that the cross-talk light is generated.
As has been described above, in the conventional equimagnification imaging device, the cross-talk light and the ghost light can not be prevented, so that the quality of an image, the contrast and the resolution deteriorate.
Thus, an equimagnification imaging device in which an aperture member is provided in the inside or the outside of the lens has been proposed. In such an imaging device, an angle range in which the light can be incident on the device and emitted from the device is limited by the aperture member. As a result, the cross-talk light and the ghost light can be reduced.
FIGS. 35A and 35B show such an equimagnification imaging device. As shown in FIG. 35A, in a roof prism or roof mirror array imaging device, an aperture member 120 prevents the light from being incident on the lens at an angle equal to or greater than a constant angle so as to prevent the cross-talk light or the ghost light from being generated.
However, in order to perfectly prevent the stray light, such as the cross-talk light or the ghost light, from being generated, each aperture on the aperture member has to be narrowed down. If each aperture is narrowed down, the light incident on the lens at the front is also limited, so that the light is not efficiently used to form an image.
In the conventional case disclosed in Japanese Patent Publication No.61-2929 described above, an image equimagnified in a direction in which lenses are arranged and a direction perpendicular to the direction in which the lenses are arranged is formed. However, in an actual line sensor having the conventional imaging device, the size of each photo element in a sub-scanning direction perpendicular to the direction in which the photo elements are arranged differs from (are greater than) the size of each photo element in the direction in which the photo elements are arranged.
Thus, in the conventional equimagnification imaging device, while the sensor unit is being moved in the sub-scanning direction, physical positions in a reading line are varied. As a result, the resolution in the sub-scanning direction deteriorates.
In addition, in the conventional case as shown in FIG. 2, when light is incident on a surface of the lens array obliquely, there is stray light between the lens array and the roof mirror array. The stray light appears on an imaging surface as ghost light or flare. As a result, an indistinct image is formed on the imaging surface.
To solve this problem, a lens array device as shown in FIG. 36 has been proposed (Japanese Laid-Open Patent Application No.5-53245). The lens array device has objective lenses, Dach surfaces and image side lenses has been proposed. A groove is formed between Dach surfaces to shade light.
However, the light incident on a boundary between the groove and the surface of the lens obliquely appears as ghost light on the imaging surface. Light reflected by the one side of the roof prism generates flare. Thus, the image formed on the imaging surface is indistinct.